Upon fine crushing of hard material, e.g. stone blocks or ore blocks, material is crushed that has an initial size of approx. 100 mm or less to a size of typically approx. 0–25 mm. Fine crushing is frequently carried out by means of a gyratory crusher. An example of a gyratory crusher is disclosed in U.S. Pat. No. 4,566,638. Said crusher has an outer shell that is mounted in a stand. An inner shell is fastened on a crushing head. The inner and outer shells are usually cast in manganese steel, which is strain hardening, i.e., the steel gets an increased hardness when it is exposed to mechanical action. The crushing head is fastened on a shaft, which at the lower end thereof is eccentrically mounted and which is driven by a motor. Between the outer and the inner shell, a crushing gap is formed into which material can be supplied. Upon crushing, the motor will get the shaft and thereby the crushing head to execute a gyratory pendulum motion, i.e., a motion during which the inner and the outer shell approach each other along a rotary generatrix and retreat from each other along another diametrically opposite generatrix.
WO 93/14870 discloses a method to set the gap between the inner and the outer shell in a gyratory crusher. Upon a calibration, a crushing head, on which the inner shell is mounted, is moved vertically upward until the inner shell comes into contact with the outer shell. This contact, which is used as a reference upon setting of the width of the gap between the inner and the outer shell, occurs at a point where the gap is most slender. In order to avoid the possibility that cast remainders or other protruding objects can affect the calibration, cast shells are subjected to a machining before they are used. This machining means that the part of the shell that can be expected to contact an opposite shell during the calibration, is made even.
It is a problem upon fine crushing of hard material by means of a gyratory crusher that a great share of the crushed material has a larger size than what was intended. For this reason, a great part of the crushed material has to be crushed one more time for achievement of the desired size.